The present invention relates generally to an improved method of forming a solid compacted pellet from powders including organic materials wherein such solid compacted pellet is used in physical vapor deposition to make an organic layer on a structure which will form part of an organic light emitting diode (OLED) display. More particularly, this invention relates to a solid compacted pellet of organic materials comprising a support member for permitting rotation of the pellet or transmitting cooling fluid during the vapor deposition process.
An organic light-emitting diode (OLED), also referred to as an organic electroluminescent device, can be constructed by sandwiching two or more organic layers between first and second electrodes.
In a passive-matrix OLED of conventional construction, a plurality of laterally spaced light-transmissive anodes, for example indium-tin-oxide (ITO) anodes, are formed as first electrodes on a light-transmissive substrate such as, for example, a glass substrate. Two or more organic layers are then formed successively by physical vapor deposition of respective organic materials from respective sources, within a chamber held at reduced pressure, typically less than 10xe2x88x923 Torr. A plurality of laterally spaced cathodes is deposited as second electrodes over an uppermost one of the organic layers. The cathodes are oriented at an angle, typically at a right angle, with respect to the anodes.
Such conventional passive-matrix OLED displays are operated by applying an electrical potential (also referred to as a drive voltage) between appropriate columns (anodes) and, sequentially, each row (cathode). When a cathode is biased negatively with respect to an anode, light is emitted from a pixel defined by an overlap area of the cathode and the anode, and emitted light reaches an observer through the anode and the substrate.
In an active-matrix OLED, an array of anodes are provided as first electrodes by thin-film transistors (TFTs) which are connected to a respective light-transmissive portion. Two or more organic layers are formed successively by vapor deposition in a manner substantially equivalent to the construction of the aforementioned passive matrix device. A common cathode is deposited as a second electrode over an uppermost one of the organic layers. The construction and function of an active matrix organic light-emitting device is described in commonly-assigned U.S. Pat. No. 5,550,066, the disclosure of which is herein incorporated by reference.
Organic materials, thicknesses of vapor-deposited organic layers, and layer configurations, useful in constructing an organic light-emitting device, are described, for example, in commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; and 4,769,292, the disclosures of which are herein incorporated by reference.
Organic materials useful in making OLED displays, for example organic hole-transporting materials, organic light-emitting materials predoped with an organic dopant, and organic electron-transporting materials can have relatively complex molecular structures with relatively weak molecular bonding forces, so that care must be taken to avoid decomposition of the organic material(s) during physical vapor deposition.
The aforementioned organic materials are synthesized to a relatively high degree of purity, and are provided in the form of powders, flakes, or granules. Such powders, flakes or granules have been used heretofore for placement into a physical vapor deposition source wherein heat is applied for forming a vapor by sublimation or vaporization of the organic material, the vapor condensing on a substrate to provide an organic layer thereon.
Several problems have been observed in using organic powders, flakes, or granules in physical vapor deposition:
(i) powders, flakes, or granules are difficult to handle because they can acquire electrostatic charges via a process referred to as triboelectric charging;
(ii) powders, flakes, or granules of organic materials generally have a relatively low physical density (expressed in terms of weight per unit volume) in a range from about 0.05 to about 0.2 g/cm3, compared to a physical density of an idealized solid organic material of about 1 g/cm3;
(iii) powders, flakes, or granules of organic materials have an undesirably low thermal conductivity, particularly when placed in a physical vapor deposition source which is disposed in a chamber evacuated to a reduced pressure as low as 10xe2x88x926 Torr. Consequently, powder particles, flakes, or granules are heated only by radiative heating from a heated source, and by conductive heating of particles or flakes directly in contact with heated surfaces of the source. Powder particles, flakes, or granules which are not in contact with heated surfaces of the source are not effectively heated by conductive heating due to a relatively low particle-to-particle contact area; and
(iv) powders, flakes, or granules can have a relatively high ratio of surface area/volume of such particles and a correspondingly high propensity to entrap air and/or moisture between particles under ambient conditions. Consequently, a charge of organic powders, flakes, or granules loaded into a physical vapor deposition source which is disposed in a chamber must be thoroughly outgased by preheating the source once the chamber has been evacuated to a reduced pressure. If outgasing is omitted or is incomplete, particles can be ejected from the source together with a vapor stream during physical vapor-depositing an organic layer on a structure. An OLED, having multiple organic layers, can be or can become functionally inoperative if such layers include particles or particulates.
Each one, or a combination, of the aforementioned aspects of organic powders, flakes, or granules can lead to nonuniform heating of such organic materials in physical vapor deposition sources with attendant spatially nonuniform sublimation or vaporization of organic material and can, therefore, result in potentially nonuniform vapor-deposited organic layers formed on a structure.
The design and performance of linear evaporation source for organic materials are described by Steven VanSlyke et al, SID 2002 Digest, pp. 886-889, 2002. The organic material in powder form is placed inside a quartz boat and heated simultaneously by bottom and top heaters. The bottom heater is used primarily to degas the powder and the top heater is operated at a temperature sufficient to vaporize the upper surface of the organic powder by radiative heating. The linear source provides a significant advantages over the conventional point source, especially thickness uniformity over a large surface area of deposition.
It is an object of the present invention to provide a method of compacting organic material adaptable for making an organic layer on a structure which will form part of an OLED display.
It is a further object of the invention to provide a method of making an organic layer from a solid compacted pellet of organic material and on a structure which will form part of an OLED display.
In one aspect, the present invention provides a method of forming a solid compacted pellet of organic materials adaptable for making an organic layer on a structure which will form part of an OLED display, comprising the steps of:
(a) providing a die defining a cavity and a first and second punches in opposite sides of the cavity and movable into such cavity, the first and second punches each having corresponding openings for receiving a cooling tube;
(b) placing organic material in a powder form inside the die cavity over the first punch and around the metal tube; and
(c) applying sufficient heat by a heating source and pressure by the punches to the organic material in the die cavity to compact the organic material into a solid compacted pellet molded about the cooling tube. comprising a metal tube at the center extending through the linear source.
In another aspect, the present invention provides a method of making a solid compacted pellet of organic material for use in a vapor deposition source, comprising the steps of:
(a) a support member for permitting rotation of the pellet, or transmitting cooling fluid, or both; and
(b) a compacted solid core of organic material molded onto and about the support member.
In another aspect, the present invention provides a method of making an organic layer from a solid compacted pellet of organic material on a structure, which will form part of an OLED display comprising the steps of:
(a) providing a solid compacted pellet of organic material comprising at least one organic host and one organic dopant;
(b) placing such a solid compacted pellet of organic material inside a heating source disposed in a physical vapor deposition chamber, circulating cold water through a cooling tube, and rotating the solid compacted pellet of organic material to exposing an area of the surface to heat for a short duration;
(c) positioning the substrate in the chamber and in a spaced relationship with respect to the heating source;
(d) evacuating the chamber to a reduced pressure; and
(e) applying heat to the top surface of the solid compacted pellet of organic material to cause at least a portion to sublime to provide a vapor of the organic materials which form the organic layer on the substrate
A feature of the present invention is that a solid compacted pellet of organic material can be compacted by the method of the invention wherein at least one organic host material and at least one organic dopant material are mixed prior to the compaction process.
Another feature of the present invention is that the method of compacting organic powder into a solid compacted pellet can be accomplished with relatively simple tools and at a location remote from a location of use of such linear source in a physical vapor deposition apparatus.
A further feature of the present invention is that the method of compacting organic powder into a solid compacted pellet substantially facilitates transfer or shipping of organic material in and between different locations.
Another feature of the present invention is that a solid compacted pellet of an OLED material can be made by the method of the present invention wherein a powder of at least one OLED host material and a powder of at least one organic dopant material are mixed or blended to provide a mixture prior to compacting the mixture into a solid compacted pellet of organic material.
Another feature of the present invention is that a solid compacted pellet of organic material comprising a host and one or more organic dopants eliminates the need of co-evaporation which require more than one evaporation source disposed inside a vacuum chamber.
Another beneficial feature of the present invention is that the solid compacted pellet of organic material is cooled at the center and a small fraction of the rotating surface is heated at a given time thereby causing uniform deposition and homogeneous composition of the organic layers.